The Truth About Calories

YOU CAN'T GO ANYWHERE WITHOUT being confronted by calories. Restaurants now print calorie counts on menus. You go to the supermarket and there they are, stamped on every box and bottle. You hop on the treadmill and watch your "calories burned" click upward.

But just what are calories? The more calories we take in, the more flab we add—and if we cut back on them, then flab starts to recede too, right? After all, at face value, calories seem to be the factor by which all foods should be judged. But if that were true, 500 calories of parsnips would equal 500 calories of Double Stuf Oreos.

Not quite. There's nothing simple about calories. Learn the distinctions and lose the lard.

Calories Fuel Our Bodies
Actually, they don't
A calorie is simply a unit of measurement for heat; in the early 19th century, it was used to explain the theory of heat conservation and steam engines. The term entered the food world around 1890, when the USDA appropriated it for a report on nutrition. Specifically, a calorie was defined as the unit of heat required to raise 1 gram of water 1 degree Celsius.

To apply this concept to foods like sandwiches, scientists used to set food on fire (really!) and then gauge how well the flaming sample warmed a water bath. The warmer the water, the more calories the food contained. (Today, a food's calorie count is estimated from its carbohydrate, protein, and fat content.) In the calorie's leap to nutrition, its definition evolved. The calorie we now see cited on nutrition labels is the amount of heat required to raise 1 kilogram of water by 1 degree Celsius.

Here's the problem: Your body isn't a steam engine. Instead of heat, it runs on chemical energy, fueled by the oxidation of carbohydrates, fat, and protein that occurs in your cells' mitochondria. "You could say mitochondria are like small power plants," says Maciej Buchowski, Ph.D., a research professor of medicine at Vanderbilt University medical center. "Instead of one central plant, you have several billion, so it's more efficient."

All Calories Are Created Equal

Not exactly

Our fuel comes from three sources: protein, carbohydrates, and fat. "They're handled by the body differently," says Alan Aragon, M.S., a Men's Health nutrition advisor. So that old "calories in, calories out" formula can be misleading, he says. "Carbohydrates, protein, and fat have different effects on the equation."

Example: For every 100 carbohydrate calories you consume, your body expends 5 to 10 in digestion. With fats, you expend slightly less (although thin people seem to break down more fat than heavy people do). The calorie-burn champion is protein: For every 100 protein calories you consume, your body needs 20 to 30 for digestion, Buchowski says. Carbohydrates and fat give up their calories easily: They're built to supply quick energy. In effect, carbs and fat yield more usable energy than protein does.

Your move
If you want to lose weight, make protein a priority at every meal.

A Calorie Ingested Is a Calorie Digested

It's not that simple

Just because the food is swallowed doesn't mean it will be digested. It passes through your stomach and then reaches your small intestine, which slurps up all the nutrients it can through its spongy walls. But 5 to 10 percent of calories slide through unabsorbed. Fat digestion is relatively efficient—fat easily enters your intestinal walls. As for protein, animal sources are more digestible than plant sources, so a top sirloin's protein will be better absorbed than tofu's.

Different carbs are processed at different rates, too: Glucose and starch are rapidly absorbed, while fiber dawdles in the digestive tract. In fact, the insoluble fiber in some complex carbs, such as that in vegetables and whole grains, tends to block the absorption of other calories. "With a very high-fiber diet, say 60 grams a day, you might lose as much as 20 percent of the calories you consume," says Wanda Howell, Ph.D., a professor of nutritional sciences at the University of Arizona.

So a useful measure of calories is difficult. A lab technician might find that a piece of rock candy and a piece of broccoli have the same number of calories. But in action, the broccoli's fiber ensures that the vegetable contributes less energy. A study in the Journal of Nutrition found that a high-fiber diet leaves roughly twice as many calories undigested as a low-fiber diet does. And fewer calories means less flab.

Your move
Aim to consume at least 35 to 40 grams of fiber every day.

Exercise Burns Most of Our Calories

Not even close

Even the most fanatical fitness nuts burn no more than 30 percent of their daily calories at the gym. Most of your calories burn at a constant simmer, fueling the automated processes that keep you alive—that is, your basal metabolism, says Warren Willey, D.O., author of Better Than Steroids. If you want to burn fuel, hit the gas in your everyday activities.

"Some 60 to 70 percent of our total caloric expenditure goes toward normal bodily functions," says Howell. This includes replacing old tissue, transporting oxygen, mending minor shaving wounds, and so on. For men, these processes require about 11 calories per pound of body weight a day, so a 200-pound man will incinerate 2,200 calories a day—even if he sat in front of the TV all day.

And then there are the calories you lose to N.E.A.T., or nonexercise activity thermo-genesis. N.E.A.T. consists of the countless daily motions you make outside the gym—the calories you burn while making breakfast, playing Nerf football in the office, or chasing the bus. Brandon Alderman, Ph.D., director of the exercise psychophysiology lab at Rutgers University, says emerging evidence suggests that "a conscious effort to spend more time on your feet might net a greater calorie burn than 30 minutes of daily exercise."

Low-calorie Foods Help You Lose Weight

Not always

Processed low-calorie foods can be weak allies in the weight-loss war. Take sugar-free foods. Omitting sugar is perhaps the easiest way to cut calories. But food manufacturers generally replace those sugars with calorie-free sweeteners, such as sucralose or aspartame. And artificial sweeteners can backfire. One University of Texas study found that consuming as few as three diet sodas a week increases a person's risk of obesity by more than 40 percent. And in a 2008 Purdue study, rats that ate artificially sweetened yogurt took in more calories at subsequent meals, resulting in more flab. The theory is that the promise of sugar—without the caloric payoff—may actually lead to overeating.

"Too many people are counting calories instead of focusing on the content of food," says Alderman. "This just misses the boat."